For electroluminescent fluorescent particles, in many cases, preferably, zinc sulfide or calcium sulfide is used, or calcium strontium sulfide, zinc selenide, zinc sulfide selenide, or the like is used. It is thought that light emission is attained by a mechanism in which copper sulfide is formed inside particles for forming electron-hole pairs when a voltage is applied; electrons-holes are generated when an electric field is applied between copper sulfide and a matrix crystal, and they emit light by recombination. Accordingly, in a case, for example, of zinc sulfide crystal, which is most widely employed, decreasing the particle size thereof has been thought to be disadvantageous to attain higher luminance and higher durability based on high crystallinity. That is, to obtain high crystallinity, generally, a wurtzite structure is once formed at a high temperature of over 1,000° C. in the presence of a sufficient flux, to grow particles having an average particle diameter of 20 μm or greater, and then the particles are partially converted to a sphalerite structure, by a second sintering at a low temperature, or the like, to form stacking fault. It has been thought that copper sulfide is formed along dislocation on the stacking fault plane. It is known that an electroluminescent fluorescent substance having high luminance has a large particle diameter, and also stacking fault (for example, see U.S. Pat. No. 5,643,496, and JP-A-06-306355 (“JP-A” means unexamined published Japanese patent application)). For the above reason in the production, it is particularly difficult to attain a small particle, of diameter 20 μm or less, at high yields. For particles having small size, particles may be decreased in size by lowering the temperature or decreasing the amount of a flux. However, such particles have low crystallinity and low luminance, or they are degraded in durability, so that it has not been possible to produce any excellent fluorescent substance.
On the other hand, with regard to a method to improve an electroluminescent fluorescent substance in durability, there is disclosed a method using a specific dopant. For example, Japanese Patent No. 2994058 describes a technique of doping with gold, to improve durability; JP-A-11-172245 and JP-A-2000-136381 describe techniques of doping with cesium, to improve durability; Japanese Patent No. 2994058 describes a technique of doping with beryllium, to improve durability; and JP-A-2000-178551 and JP-A-2002-053854 describe techniques of sintering zinc sulfide in an antimony or bismuth atmosphere, to improve durability. However, the effects of these techniques are insufficient, and even a decrease in initial luminance has been sometimes involved.
When small-size particles are used as an EL fluorescent substance, therefore, such particles are expected to have an advantage, for example, in forming a uniform fluorescent substance coating layer and decreasing the same in thickness. Due to a decrease in luminance and degradation of durability, however, it has been considered that no preferable electroluminescent fluorescent substance can be obtained.